Integer handling

Hi David,

let me first thank you thanks for your nice work! Your audiolab module has proven quite useful for me in trying out some dsp related stuff in python.

While trying to generate some bit exact testdata I ran into problems with saving this data via audiolab on my 64bit machine. The problem seems to be that audiolab assumes np.int == np.int32 in the write_frames() method which isn't true on my 64bits installation of python.

I looked into the code and found a solution which works for me and should solve that problem. I'd be happy if you could take a look on this pull request and tell me if you see any problems with the changes I made.

Cheers,
Matthias